How to Handle Unused Op Amp Sections?

[Carey Fisher - NCS]

I have a design where I am using 3 of the four sections of a TL074 Quad Op
Amp. What should I do with the unused pins, especially the inputs, on the
fourth Op Amp to prevent it from causing any problems?
Thanks,

 

[John Woodgate]

I read in sci.electronics.design that Carey Fisher - NCS
outh.net>) about 'How to Handle Unused Op Amp Sections?', on Wed, 28 Jan
2004:

I have a design where I am using 3 of the four sections of a TL074 Quad Op
Amp. What should I do with the unused pins, especially the inputs, on the
fourth Op Amp to prevent it from causing any problems?
Thanks,

Ground both of the inputs. Leave the output unconnected.

 

[Carey Fisher - NCS]

I read in sci.electronics.design that Carey Fisher - NCS
outh.net>) about 'How to Handle Unused Op Amp Sections?', on Wed, 28 Jan
2004:

Ground both of the inputs. Leave the output unconnected.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Thanks John, DC Ground?

 

[Tim Dicus]

May be bad advice- connect IN(+) to power supply common-mode, 0V for
bipolar and V+/2 for single, connect OUT to IN(-).

Hi Fred,

I use your way. Some op-amp inputs will be out of common-mode range at ground. May cause latching or oscillation.

Tim

 

[Fred Bloggs]

Thanks John, DC Ground?

May be bad advice- connect IN(+) to power supply common-mode, 0V for
bipolar and V+/2 for single, connect OUT to IN(-).

 

[John Woodgate]

I read in sci.electronics.design that Carey Fisher - NCS
uth.net>) about 'How to Handle Unused Op Amp Sections?', on Wed, 28 Jan
2004:

Thanks John, DC Ground?

I'm not quite sure what you mean. You wire them directly to whatever is
'ground' for the sections of the op-amp that you are using.

 

[Frank Bemelman]

May be bad advice- connect IN(+) to power supply common-mode, 0V for
bipolar and V+/2 for single, connect OUT to IN(-).

I often connected such op amp follower to one of the possible
interesting nodes of the circuit, and label it's output TPxx

 

[John Woodgate]

I read in sci.electronics.design that Fred Bloggs <[email protected]>

May be bad advice- connect IN(+) to power supply common-mode, 0V for
bipolar and V+/2 for single, connect OUT to IN(-).

Bad advice!

You think the OP will understand 'power supply common mode'? 'Bipolar'
in that context, in which a CMOS op-amp can be 'bipolar'? 'Single' when
you really mean 'single supply'?

You really DON'T need to keep asserting your superior skills, especially
when that is very likely to totally confuse an enquirer.

Connecting an unused section as a follower is OK *as well*.

 

[John Larkin]

Hi Fred,

I use your way. Some op-amp inputs will be out of common-mode range at ground. May cause latching or oscillation.

Tim

Some opamps also draw a lot of current when they go open-loop railed.
Some cheapie LM324 types will go bonkers in their shared current
sources, so one channel railing will mess up the others.

John

 

[Jim Thompson]

I read in sci.electronics.design that Fred Bloggs <[email protected]>



Bad advice!

You think the OP will understand 'power supply common mode'? 'Bipolar'
in that context, in which a CMOS op-amp can be 'bipolar'? 'Single' when
you really mean 'single supply'?

You really DON'T need to keep asserting your superior skills, especially
when that is very likely to totally confuse an enquirer.

Connecting an unused section as a follower is OK *as well*.

Is this "pick-on-Fred" day ?

His "May be bad advice..." seems perfectly correct to me. Or are you
being "bipolar" today ?

...Jim Thompson

 

[Walter Harley]

I have a design where I am using 3 of the four sections of a TL074 Quad Op
Amp. What should I do with the unused pins, especially the inputs, on the
fourth Op Amp to prevent it from causing any problems?

I always connect - to OUT (unity gain follower) and connect + to ground. I
figure shorting the two inputs together is asking for trouble because I
don't know what the offset voltage is, or whether external conditions are
going to affect it, so the output is unpredictable. Unpredictable voltages
scare me.

Does anyone know of any reason *not* to use the unity gain follower? (We're
talking about a unity-gain compensated amp, of course.)

 

[John Woodgate]

I read in sci.electronics.design that Tim Dicus <[email protected]>

I use your way. Some op-amp inputs will be out of common-mode range at
ground. May cause latching or oscillation.

You are using a different definition of 'ground'.

 

[John Woodgate]

I read in sci.electronics.design that Jim Thompson
4ax.com>) about 'How to Handle Unused Op Amp Sections?', on Wed, 28 Jan
2004:

Is this "pick-on-Fred" day ?

Not particularly. I do feel that his posts to newbies are more intended
to impress than inform.

His "May be bad advice..." seems perfectly correct to me. Or are you
being "bipolar" today ?

Well, there's 'correct' and 'correct. It depends on what assumptions you
make, like assuming that when I say 'ground', I mean 'a point that
drives the device outside its CM range'.

 

[John Woodgate]

I read in sci.electronics.design that Walter Harley
..com>) about 'How to Handle Unused Op Amp Sections?', on Wed, 28 Jan
2004:

Does anyone know of any reason *not* to use the unity gain follower?
(We're talking about a unity-gain compensated amp, of course.)

AHA! Precisely.

 

[NCSRadio]

I read in sci.electronics.design that Carey Fisher - NCS
uth.net>) about 'How to Handle Unused Op Amp Sections?', on Wed, 28 Jan
2004:


I'm not quite sure what you mean. You wire them directly to whatever is
'ground' for the sections of the op-amp that you are using.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Well, I'm gonna connect Out to In(-) and In(+) to Vcc/2 (using single
supply). Might even find a use for a unit gain buffer (thanks Frank).

--
Carey Fisher
Tag Lines Irritate the Stew Out of Me!
Tag Lines Irritate the Stew Out of Me!
Tag Lines Irritate the Stew Out of Me!
Tag Lines Irritate the Stew Out of Me!

 

[NCSRadio]

I read in sci.electronics.design that Fred Bloggs <[email protected]>



Bad advice!

You think the OP will understand 'power supply common mode'? 'Bipolar'
in that context, in which a CMOS op-amp can be 'bipolar'? 'Single' when
you really mean 'single supply'?

You really DON'T need to keep asserting your superior skills, especially
when that is very likely to totally confuse an enquirer.

Connecting an unused section as a follower is OK *as well*.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

This OP DID understand!

 

[John Larkin]

I always connect - to OUT (unity gain follower) and connect + to ground. I
figure shorting the two inputs together is asking for trouble because I
don't know what the offset voltage is, or whether external conditions are
going to affect it, so the output is unpredictable. Unpredictable voltages
scare me.

Does anyone know of any reason *not* to use the unity gain follower? (We're
talking about a unity-gain compensated amp, of course.)

That's the thing to do. If supplies are bipolar, use ground on the +
input; if it's a single-supply system, use anything above zero that's
within the common-mode range, which could include another signal if
that's most convenient.

John

 

[Tim Dicus]

I read in sci.electronics.design that Tim Dicus <[email protected]>


You are using a different definition of 'ground'.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Maybe negative supply voltage would have been better?

Tim

 

[John Woodgate]

I read in sci.electronics.design that Tim Dicus <[email protected]>

Maybe negative supply voltage would have been better?

That would make your comment about latching or oscillation correct. But
I didn't mean 'negative supply', I meant 'ground', which may also be
negative supply but could be positive supply or neither. I did think
about saying 'signal common', but then I thought about balanced signals
and whether the OP could misinterpret that term.

 

[Tim Dicus]

I read in sci.electronics.design that Tim Dicus <[email protected]>

That would make your comment about latching or oscillation correct. But
I didn't mean 'negative supply', I meant 'ground', which may also be
negative supply but could be positive supply or neither. I did think
about saying 'signal common', but then I thought about balanced signals
and whether the OP could misinterpret that term.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I realize what you meant now. I had a single-side voltage supply in mind. I don't know why.

But I still like the output to IN- idea. I believe it may be more stable.

Tim